A new file system called a Linear Tape File System (LTFS) has recently been developed as a new use for tape.
LTFS is software (S/W) which works with hardware (H/W) resources to enable tape to be accessed via the file system interface.
LTFS is realized by storing data on tape cartridges using the LTFS format, which has open specifications, and has been adopted by many companies other than IBM (IBM, the IBM logo, and ibm.com are trademarks or registered trademarks of International Business Machines Corporation, registered in many jurisdictions worldwide).
The LTFS format specifies that the tape built into the tape cartridge be divided into two partitions: an index partition (IP) and a data partition (DP).
Here, metadata such as allocation data for a file is recorded in the index partition, and file data is primarily recorded in the data partition.
The following is background information related to this recording process.
File allocation information is frequently updated, but data is always being appended so that the tape becomes a so-called sequential access device. When data is recorded to a single partition, the allocation information is always recorded at the end.
Because the data at the end has to be read when a tape cartridge is mounted, the mounting process is time consuming.
In LTFS, metadata is overwritten in the beginning portion of the index partition when a tape cartridge is unmounted. As a result, it is always possible to read the metadata from the index partition when a tape is mounted.
More precisely, metadata is also written to the data partition. Even though this takes time, the metadata recorded in the data partition can then be used to restore a mounted tape cartridge when metadata cannot be updated in the index partition due, for example, to a sudden power interruption.
A mechanism is also used to enable storage of a small amount of data in the index partition. Data to be read during the mounting process can then be retrieved rapidly from the index partition by recording the data not only in the data partition but also in the index partition.
FIG. 1 is a diagram showing an example of information recorded on a tape cartridge.
In this example, a separately designated file (File B) and the most recent metadata (Metadata 3) are recorded in the index partition.
Metadata (Metadata 1, Metadata 2, Metadata 3) is also recorded in the data partition along with the actual file data (File A, File B, File, C, File D).
Here, Metadata 1 and Metadata 2 in the data partition is old metadata. Because this information is basically appended to the tape, it is stored without performing any overwriting.
The timing for writing this metadata to the data partition can be established explicitly by an application (for example, by calling FlushFileBuffers( ) which is a standard API in Windows (a trademark of Microsoft Corporation, registered in many jurisdictions worldwide)), can be established based on the timing for closing files in IBM LTFS, or can be established to occur once a predetermined period of time has elapsed.
There are two basic versions of LTFS; LTFS SDE (Linear Tape File System Single Drive Edition) in which a single tape cartridge is inserted into a single tape drive, and LTFS LE (Linear Tape File System Library Edition) in which a plurality of tape cartridges are inserted into a tape library.
In LTFS LE, each LTFS-formatted tape cartridge incorporated into a tape library is expressed as a directory, and the data in each tape cartridge can be accessed as files.
FIG. 2 is a diagram showing a view of directories in LTFS LE as an example in which a tape library including inserted tape cartridges AAA000, BBB000 and CCC000 have been mounted using LTFS LE.
In LTFS LE, the directories (dir1, dir2, dir3) created in tape cartridge CCC000 can be viewed as subdirectories of directory CCC000.
When files are read and updated in a tape cartridge using LTFS LE, a tape cartridge is mounted in a tape drive, and the data is accessed on the tape cartridge.
The number of tape drives incorporated into a tape library is generally smaller than the number of tape cartridges.
Therefore, when a tape cartridge is to be mounted in a tape drive, a tape cartridge in use has to be unmounted if there are no free tape drives, and the tape cartridge to be accessed is mounted.
FIG. 3 is a diagram showing the unmounting and mounting flow when reading and writing occur.
In LTFS LE, the unmounting and mounting operations are concealed from the user, as shown in FIG. 3. If necessary (in accordance with the access request), the tape drives are assigned using an LRU-based algorithm, and tapes are unmounted and mounted in a manner not visible to the user.
When LTFS LE is used, a tape library with a large storage capacity can be used as ordinary disk storage. In this case, the LTFS LE may be shared via a network to create NAS.
Because information is recorded on tape cartridges using the LTFS format, which is an open standard, LTFS LE can be easily used to remove tape cartridges from a tape library and read them using different systems.
When data is moved to a remote site as an emergency precaution or to allow data to be shared at multiple locations, tape cartridges can be removed from a tape library and physically moved without having to establish an expensive broadband network.
Because it is less expensive to use NAS with HDDs, LTFS LE has attracted attention as a means of handling large quantities of data.
When LTFS LE is used as NAS and tape cartridges are removed and used if necessary, it is important to prevent data tampering in the case of removed data cartridges.
One method of preventing data tampering in LTO tape drives and TS1140 tape drives is a mechanism referred to as “WORM (Write Once Read Many)”.
When WORM tape cartridges are used (referred to below as “WORM cartridges”), just as with CD-R, data once written cannot be updated or deleted.
Therefore, there is demand for the use of WORM cartridges in LTFS LE as a way of preventing data tampering.
However, LTFS currently does not support WORM cartridges for the following two reasons.                1. Fifth-generation and sixth-generation Linear Tape-Open (LTO) tape drives and TS1140 tape drives supported by LTFS do not support the creation of partitions in WORM cartridges.        2. Even if a tape drive were to support the creation of these partitions, metadata cannot be overwritten in the index partition of a WORM cartridge. It is appended.        
Therefore, metadata is frequently appended to the index partition as a sequence of operations is performed such as writing a small number of files, unmounting a cartridge, and writing a small number of files immediately after mounting the cartridge.
Usually, less area is assigned to the index partition than to the data partition in order to be able to store more data. However, when data is frequently appended to the index partition, the capacity of the index partition may be exhausted even though more space is still available in the data partition, and the tape may no longer be updated.
Because the cartridge mounting and unmounting operations are concealed from the user in LTFS LE, mounting and unmounting may occur frequently (simply by repeatedly accessing files in different directories) without the user being aware that this is occurring.
As a result, the index partition may be updated many times without the user being aware of this, and a large amount of the index partition may be consumed.
Therefore, unnecessary appending of data has to be eliminated in order to reduce consumption of capacity in the index partition.
It is not technically difficult to support (1) above, because a WORM cartridge could simply be partitioned in advance, for example, prior to shipment.
Shipment of LTFS-formatted cartridges is being considered for cartridges intended for use in LTFS.
A WORM cartridge may be prepared or formatted by dividing the built-in tape into two WORM partitions, for recording the history of a plurality of files and metadata (including the allocation of one or more recorded files) in the one WORM data partition, and for recording metadata in the other WORM index partition.
A method has been developed for (2) above by applying WORM using only a data partition. This method is explained in detail in an application filed by the present applicant (see Patent Literature 1).
However, a problem remains with applying WORM using only a data partition as disclosed in Patent Literature 1 because there is no index partition for WORM, and tampering with index partition data cannot be detected.